And a few pics of our first build and our (very basic) system in the early days:

Loaded with PD ET211S (750W RMS, 4ohm - no other specs available as PD seemed to have incinerated any of the specs for speakers that were manufactured prior to when the company changed hands several years ago.)

2 years, a wedding and a new flat later, and I'm finally ready to replace my 3 cubo 18's with 2 Cubo Subs! My question really is driver choice... I'd previously asked about the RCF LF18X401 which seemed like a good choice, but what I'm curious about is what are some of the 'best' driver options, for maximum SPL and lowest Hz. I've seen the B & C 18TBW100 has been recommended, I play mostly Dub/Dubstep and hoping I can replace my 3 Cubo 18's (eminence Sigma Pro 2 loaded) with 2 Cubo subs that will outperform them in Hz, and at least equal them in 'volume'.
Thanks as always for your time!

Cubo 18 will typically have a bit more output between 50 - 80 Hz than Cubo Sub, likewise Cubo Sub will have more output below 50 Hz.
The 18TBW100 and the LF18X401 are similar drivers, from different brands.
The RCF is a bit more towards the Sigma Pro (Qes/ Vas wise) so that
would be my first choice. If you're able to feed each driver, twice what
you're giving a single Sigma Pro, you should get close to similar
output. Some of the best drivers out there are simply a single step up from here, drivers like the B&C 18DS115 and 18SW115
for example.

The RCF LF18N405 is a pretty all round driver, T/S-parameter wise but it's definitely designed for use in sub bass enclosures, excursion wise. In that last regard, it makes most sense to put it in something like Cubo Sub, so that it can utilize it's excursion capabilities by focusing on the bottom octave of music. It's not to efficient so I expect it to be pretty flat in it's working range.

There seems to be very little discussion on this thread about the 21" version of this design - mostly people asking about 18" drivers and construction.

Perhaps I am one of the few to have made one, so this New Year (just passed) we decided to put two of them through their paces. They seemed to be doing pretty well for a good few hours and then....panic...everyone's coming up to me like....there's no bass....oh dear!

Had a look at the drivers after the event - burnt voice coil...no surprise. Admittedly we were using 21" drivers from Bishopsound which rather suspiciously were missing a few key parameters i.e. voice coil size etc. and you had warned us in a previous post that it might be a little risky putting a bridged CA12 amp (2800W into 4Ohm) to power this 2000W driver. Anyhow, we live and learn eh...!

So, got a couple of questions for ya:

i.) Do you think the fact that we built handle boxes into the chamber (see image, green & purple) might have meant that the reduced volume of the chamber was providing too much resistance to the movement of the cone (this resistance is necessary right - to ensure that the cone doesn't suffer from over-excursion..but it needs to be a cone (and voice coil) that can handle this right..?)

ii.) The fact that the handles are placed in slightly different positions within the chamber meant that the two drivers were slightly out of phase and therefore not vibrating in unison i.e. slightly working against each other (I imagine your answer to this will probably be "it's negligible")

iii.) Considering the considerable expense of purchasing new (half decent) drivers, might it be worth considering making another baffle which could accomodate an 18" driver & screwing it onto the existing baffle? Would this cabinet still perform well in that case or would it not be able to move enough air for such a large enclosure...

iii.) Finally - I had been in touch a while ago about building an extension to our two cubo subs - problem is when you put them both together mouth-to-mouth, the horn section suddenly expands due to the heavy bracing on the bottom panel - I am aware that any sudden expansion in the horn should be avoided. Do yo think that making a triangular brace (see image, orange) might help to alleviate this problem...would it make any difference?

Many thanks for your advice in advance - know that is a lot of questions and you are a busy man.

We are quite keen to keep using these cabinets rather than make something like a Super Bass Horn (Startec) just a question of finding what works best.

P.S. if we were going to load them with 18" drivers we would look at the Fane Colossus 18XB or replacement 21" drivers, ﻿the RCF LF21X451.